The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 to Stemme.
There are various known types of thermal inkjet (bubblejet) printhead devices. Two typical devices of this type, one made by Hewlett Packard and the other by Canon, have ink ejection nozzles and chambers for storing ink adjacent the nozzles. Each chamber is covered by a so-called nozzle plate, which is a separately fabricated item and which is mechanically secured to the walls of the chamber. In certain prior art devices, the top plate is made of Kapton™ which is a Dupont trade name for a polyimide film, which has been laser-drilled to form the nozzles. These devices also include heater elements in thermal contact with ink that is disposed adjacent the nozzles, for heating the ink thereby forming gas bubbles in the ink. The gas bubbles generate pressures in the ink causing ink drops to be ejected through the nozzles.
Before printing, the chambers need to be primed with ink. During operation, the chambers may deprime. If the chamber is not primed the nozzle will not eject ink. Thus it is useful to detect the presence or absence of ink in the chambers. However, the microscopic scale of the chambers and nozzles makes the incorporation of sensors difficult and adds extra complexity to the fabrication process.
The resistive heaters operate in an extremely harsh environment. They must heat and cool in rapid succession to form bubbles in the ejectable liquid, usually a water soluble ink. These conditions are highly conducive to the oxidation and corrosion of the heater material. Dissolved oxygen in the ink can attack the heater surface and oxidise the heater material. In extreme circumstances, the heaters ‘burn out’ whereby complete oxidation of parts of the heater breaks the heating circuit.
The heater can also be eroded by ‘cavitation’ caused by the severe hydraulic forces associated with the surface tension of a collapsing bubble.
To protect against the effects of oxidation, corrosion and cavitation on the heater material, inkjet manufacturers use stacked protective layers, typically made from Si3N4, SiC and Ta. In certain prior art devices, the protective layers are relatively thick. U.S. Pat. No. 6,786,575 to Anderson et al (assigned to Lexmark) for example, has 0.7 μm of protective layers for a ˜0.1 μm thick heater.
To form a vapor bubble in the bubble forming liquid, the surface of the protective layers in contact with the bubble forming liquid must be heated to the superheat limit of the liquid (˜300° C. for water). This requires that the heater and the entire thickness of its protective layers be heated to 300° C. If the protective layers are much thicker than the heater, they will absorb a lot more heat. If this heat cannot be dissipated between successive firings of the nozzle, the ink in the nozzles will boil continuously and the nozzles will stop ejecting. Consequently, the heat absorbed by the protective layers limits the density of the nozzles on the printhead and the nozzle firing rate. This in turn has an impact on the print resolution, the printhead size, the print speed and the manufacturing costs.
Attempts to increase nozzle density and firing rate are hindered by limitations on thermal conduction out of the printhead integrated circuit (chip), which is currently the primary cooling mechanism of printheads on the market. Existing printheads on the market require a large heat sink to dissipate heat absorbed from the printhead IC.
Inkjet printheads can also suffer from nozzle clogging from dried ink. During periods of inactivity, evaporation of the volatile component of the bubble forming liquid will occur at the liquid-air interface in the nozzle. This will decrease the concentration of the volatile component in the liquid near the heater and increase the viscosity of the liquid in the chamber. The decrease in concentration of the volatile component will result in the production of less vapor in the bubble, so the bubble impulse (pressure integrated over area and time) will be reduced: this will decrease the momentum of ink forced through the nozzle and the likelihood of drop break-off. The increase in viscosity will also decrease the momentum of ink forced through the nozzle and increase the critical wavelength for the Rayleigh Taylor instability governing drop break-off, decreasing the likelihood of drop break-off. If the nozzle is left idle for too long, the nozzle is unable to eject the liquid in the chamber. Hence each nozzle has a maximum time that it can remain unfired before evaporation will clog the nozzle.